1. Field of the Invention
This invention relates to a driving apparatus for solid state image sensors, which is used for an electronic still camera.
2. Description of the Prior Art
A solid state image sensor, such as a CCD is used as a light receiving element for an electronic still video camera. An electronic still video camera of this kind is adapted to convert optical information into an electric signal and store the resultant electric signal on an information recording medium, such as a video floppy. Accordingly, this video camera is advantageous unlike a silver halide film camera in that it does not require developing and is capable of transferring image information to a remote area.
In order to drive a solid state image sensor in such an electronic still camera, carrying out a photographing operation (including the exposing of a film and the transferring of electric charges) corresponding to one field or one frame only may meet the purpose unlike the case of a video camera. Unnecessary electric charges also occur in the light receiving element of CCD even before the starting of a practical exposure action. The generation of such electric charges is noticeable, especially, during an operation of an electronic shutter. Therefore, it is necessary that the unnecessary electric charges be swept out prior to the starting of the exposure.
A shutter button consists normally of a two-step push switch including S1, S2 (not shown), and is constructed so that S1 is turned on when the switch is pressed lightly, and so that both S1 and S2 are turned on when the switch is pressed deep. When S1 is turned on, a controlling microcomputer sends out instructions for the auto-focusing (AF), exposure controlling (AE) and sweeping of unnecessary electric charges to a range finding circuit, a photometric circuit and the solid state image sensor driving apparatus, respectively. The instructions for AF, AE and sweeping of unnecessary electric charges are sent out in repetition periodically until S2 has been turned on. When S2 is turned on, the microcomputer gives out an instruction for the exposure controlling to the solid state image sensor driving apparatus.
However, it is necessary in the above structure that the microcomputer has a special port for giving out an instruction for the sweeping of unnecessary electric charges. It is also necessary that the microcomputer directly gives out an instruction for sweeping out unnecessary electric charges with the instructions for AF and AE while S1 of the shutter button is pressed. Since the instruction for sweeping out the unnecessary electric charges is given out with the instructions for AE and AF, no problems arise in a regular case. However, when the operations are timed in a certain manner with an exposure instruction given out in a stage in which an unnecessary electric charge sweeping instruction has not yet been given out, unnecessary electric charges remain on the light receiving element in the solid state image sensor. Consequently, the unnecessary electric charges and the electric charges occurring due to the exposure are mixed to cause the quality of the image to decrease.
Such an electronic still camera is constructed so that it is suitable to take still pictures one by one. Namely, a horizontal synchronizing signal (H.sub.SYNC) is reset (H reset) at the exposure starting time and the exposure finishing time. When the exposure and reading are carried out alternately and continuously under such conditions, H.sub.SYNCs become discontinuous due to the resetting thereof, so that the image on the picture frame of the monitor is disarranged and not clearly seen. Such continuous exposure/reading operations are carried out when regulation and evaluation are made but, unless the image on the picture frame of the monitor becomes stable, regulation and evaluation cannot be made.
Such an electronic still camera is to take photographs continuously at a high speed in some cases. During such a continuous photographing operation, time is needed to sweep out unnecessary electric charges, expose a film and record the information, and these actions are repeated, so that there is a limit to the improving of the continuous photographing speed. Therefore, the continuous photographing speed cannot be improved satisfactorily. It is also necessary in a high-speed continuous photographing operation to synchronize the driving of the solid state image sensor with the rotating of the video floppy, and much time is spent in carrying out this operation.
FIGS. 14A-14G are timing charts for the operations of a driving apparatus for a solid state image sensor, which is used in a conventional electronic still camera. These timing charts are based on a case where a frame interline CCD is used. A case where the exposure of a film is carried out initially with a signal which is obtained by the exposure read out thereafter will be described.
In order to set the phases of rotation of the solid state image sensor driving apparatus and video floppy in agreement with each other, the resetting [the H resetting (the resetting of a horizontal synchronizing signal) and the V resetting (the resetting of a vertical synchronizing signal) ] is done (FIGS. 14D and 14E .circle.1 ) 7H (horizontal scanning period) after the rising of the rear end of a PG pulse consiting of a reference signal of the rotation of the video floppy. As a result, the PG pulse is changed from a high level to a low level and passed through a gate, so that a H resetting signal and a V resetting signal are formed. The falling of a trigger signal designates (FIG. 14A .circle.2 ) the starting of exposure, and a high-speed transfer pulse is generated immediately thereafter in a .phi. I signal (FIG. 14B), which is one of CCD image area drive pulses to sweep out the unnecessary electric charge on a vertical transfer portion of the solid state image sensor. A sensor gate (SG) pulse (FIG. 14B .circle.3 ) is then generated in the .phi.I signal to sweep out the unnecessary electric charge on a light receiving element in the solid state image sensor by the high-speed transfer pulse. Then, the electric charge is accumulated in the light receiving element by a SG pulse .circle.3 '. The rising of the trigger signal designates (FIG. 14A .circle.4 ) the stopping of the exposure, and a high-speed transfer pulse is generated immediately thereafter in the .phi.I signal to sweep out the electric charge generated on a vertical transfer passage for the solid state image sensor. The signal charges accumulated in the light receiving element are then transferred (FIG. 14B .circle.5 ) to a storage portion by the high-speed transfer pulse, after the electric charge of the light receiving element is transferred to V-CCD by the sensor gate pulse. Accordingly, the time between .circle.3 ' and .circle.5 is used practically as an exposure period. Then, the PG pulse is changed to a low level at the following SG pulse in the two SG pulses and passed through the gate, so that a H resetting pulse and a V resetting pulse are formed. As has been described above, the resetting is done again so that V.sub.SYNC occurs 7 H after the rise of the rear edge of the PG pulse (FIGS. 14D and 14E .circle.6 ). Then, the reading of the electric charges generated due to the exposure from the solid state image sensor is started at the instant .circle.7 , so that the signal read is recorded in a recorder 3.
If the exposure is completed near a period V.sub.SYNC even in the case where the operations timed as mentioned above are carried out, an image cannot be outputted from the solid state image sensor. Specifically, if the period of the high-speed transfer pulse between .circle.4 and .circle.5 in FIG. 14 overlaps the period V.sub.SYNC and a period of 10 H direct before the V.sub.SYNC the start .circle.7 of the signal reading may not be carried out properly, because a signal reading pulse generating circuit cannot be effected suitably when the period of the high-speed transfer pulse between .circle.4 and .circle.5 overlaps a counting in a V counter which becomes a standard of generation of V.sub.SYNC.
Therefore, it is necessary that a system controlling microcomputer in the electronic still camera constantly monitors SYNC signals to control the operations so that the exposure is not completed during the period V.sub.SYNC. Consequently, if the operations are timed in a certain manner, the waiting time occurs, so that the starting and finishing of exposure cannot be done at the desired time. Due to this waiting time, the continuous photographing speed cannot be increased.
The necessity that the resetting is done before and after exposure to match the phases of rotation of the solid state image sensor driving apparatus and video floppy also proves to be troublesome.
In order to photograph an object by using this kind of electronic still video camera with a strobe operated, it is necessary for the following reasons that the exposure be controlled with a high accuracy. In the case where CCD is used, a bright portion of an image is whitened if the exposure has exceeded an optimum level even slightly, and a dark portion of the image blackened if the exposure has become lower even slightly than the optimum level. In the case where a conventional silver halide film is used, an image formed thereon can be corrected during the development or printing even when the exposure deviates a little from an optimum level. Accordingly, in a conventional silver halide still camera, an exposure control operation can be carried out comparatively simply (called flashmatic control) by initially determining a distance of an object by an auto focusing means (automatic focus regulating means) on the basis of the equation, Guide number=Distance.times.Aperture, and then determining aperture on the basis of the same equation. Moreover, the distance may be set in eight steps with respect to the range of .infin. (infinite point)-1 m.
In the case of an electronic still camera using CCD as a light receiving element, an optimum exposure control operation cannot be carried out by the flashmatic control method as mentioned above since the latitude of the CCD is narrow. Therefore, it is necessary in the electronic still camera that the exposure be controlled with a high accuracy. For example, a method of controlling the rate of emission of a light regulating strobe in use is employed.
FIG. 15 is a block diagram of an exposure control system for a conventional electronic still camera. When a strobe control signal (emission starting signal) is inputted into an emission controller 41, the emission controller 41 operates a strobe 42 to emit light therefrom. When the light is emitted from the strobe 42, an object 43 is irradiated therewith, and a reflected light from the object 43 enters a light receiving element 45 through a light receiving lens 44. In an integrating circuit 46, a photoelectrically converted output from the light receiving element 45 is integrated simultaneously with the emission of light from the strobe 42. When an output from the integrating circuit 46 has reached a light regulating level which is determined by the sensitivity of CCD and a selected aperture, a stop signal is applied from a comparator 47 to the emission controller 41. Consequently, the emission controller 41 stops the light emitting action of the strobe 42.
FIG. 16 is a graph showing the conversion characteristics of the rate of emission of the strobe in this exposure ontrol system. Referring to FIG. 16, the longitudinal axis represents the light quantity of the strobe and the lateral axis time t. At an instant t.sub.1, a strobe control signal is generated, and the quantity of light of the strobe increases suddenly as shown in FIG. 16. When an integrated value from the integrating circuit 46 has reached a light regulating level at an instant t.sub.s, the emission of light from the strobe 42 stops. The hatched portion between these instants of this graph represents an actual quantity of the emitted light. A broken line in FIG. 16 is an emission curve of the strobe 42 with respect to the time in which the light is fully emitted. Let t.sub.2 equal an instant at which the quantity of the emitted light becomes zero. An exposure control system in which t.sub.s is earlier than t.sub.2 can be suitably used.
A solid state image sensor having an photo sensor, a transfer unit and a gate for transferring the accumulated electric charges in the photo sensor to the transfer unit may employ a method of regulating exposure by setting variable a period in which the gate is turned on, applying a gate pulse to the gate when a proper exposure has been attained, to turn on the gate, and reading out the accumulated electric charges in the photo sensor.
The strobe in use consists, for example, of a xenon tube. In order to control exposure by interrupting the emission of light from the strobe 42 as shown in FIG. 16, the construction of the circuit in the emission controller 41 becomes very complicated, and a difference occurs between the time at which an emission stopping signal is outputted and that at which the emission of light practically stops. Therefore, it is difficult to turn off the strobe with a high accuracy in the midst of the emission of light therefrom, and, especially, it is very difficult to turn off the strobe with a high accuracy in a rising part of the emission of light therefrom. Consequently, a photograph obtained in a photographing operation using even an automatically light regulating strobe with an aperture opened, especially, for the object near the camera, has a whitened image in many cases. Moreover, due to the complicated circuit structure, the dimensions of the system increases to cause the manufacturing cost of the apparatus becomes high.
In a method of regulating exposure by changing the time at which a pulse is applied to a gate in a solid state image sensor having a photo sensor, a gate and a transfer unit, a pulse for turning on the gate is applied thereto at an instant at which a proper exposure is attained, to read the signal charges. Accordingly, the controlling of the exposure during the time in which the exposure is in a proper level is done reliably. However, since the object is positioned in the distance, a proper exposure is not attained in some cases. In order to prevent this, it is necessary that a system controlling microcomputer outputs an emission stopping signal. This makes it necessary to carry out a control operation using a special port provided in the microcomputer, and causes the construction of the apparatus and an exposure control operation to become complicated.